Electrical circuit

ABSTRACT

An electrical circuit for connection to a high tension lead which is connected to a spark plug of a spark ignition internal cumbustion engine. The circuit comprises a capacitor (11) the capacitance of which is such that, if a high voltage pulse is applied to the high tension lead, the voltage developed across the capacitor and the charge stored by the capacitor are sufficient to initiate and sustain and ignition spark. The capacitor may be voltage dependent to achieve an optimised spark current characteristic. A resistor (12), such as a voltage dependent resistor, and a voltage controlled discharge device (13) may be connected in parallel with the capacitor.

The present invention relates to an electrical circuit for use in aspark-ignition internal combustion engine.

In a conventional spark-ignition internal combustion engine, spark plugsare connected to a high voltage supply such as an iginition coil througha distributor. The distributor periodically closes a conductive pathbetween each spark plug and the coil so as to enable a high voltage tobe applied across a gap defined by the spark plug. The high voltage issufficient to generate a spark between the electrodes of the spark plug.The distributor is connected via a single high tension lead to the coiland by respective high tension leads to each of the spark plugs.

A great deal of attention has been paid to optimising spark timing andthe conditions within the engine cylinders to which the spark plugs arefitted. Little attention has been given to the nature of the sparkitself other than to ensure that the spark is sufficiently large toreliably ignite an air/fuel mixture.

It is an object of the present invention to provide an electricalcircuit which enables the spark generated by a ignition coil tosubstantially enhance the performance of internal combustion engines.

According to the present invention there is provided an electricalcircuit for connection to a high tension lead which is connected to aspark plug of a spark ignition internal combustion engine, the circuitcomprising a capacitor the capacitance of which is such that, if a highvoltage pulse is applied to the high tension lead, the voltage developedacross the capacitor and the charge stored by the capacitor aresufficient to initiate and sustain an iginition spark, the capacitorhaving either a resistor or a voltage controlled discharge deviceconnected in parallel therewith.

Preferably, the capacitor is non-linear, for example voltage dependentsuch that its capacitance reduces with increases in applied voltage. Thecapacitor may be temperature dependent such that its capacitance reduceswith increases in operating temperature.

The resistor may be non-linear, for example voltage dependent such thatits resistance decreases with increases in applied voltage.

In embodiments having a temperature dependent capacitor and a parallelresistor, the resistor may be positioned such that heat generated in theresistor is transferred to the capacitor.

A diode may be connected in series with the capacitor.

A circuit in accordance with the present invention may be connected inseries with a spark plug of an internal combustion engine. Where thatspark plug is energised from a distributor, the circuit may be connectedeither between the distributor and the respective spark plug or betweena source of electrical energy such as a coil and the distributor.

In a system in which two or more spark plugs are to be energised fromone source, then a respective circuit may be connected in series witheach spark plug. For example, in an internal combustion engine with twospark plugs per cylinder, this arrangement would circumvent the need fordual ignition drives.

A circuit in accordance with the invention may also be used to enhancespark performance by connecting such a circuit between a high tensionlead connected to a spark plug and a source of fixed potential. Withsuch an arrangement a fuse is preferably connected in series with thecapacitor such that if the capacitor or any component in parallel withthe capacitor fails to a low impedance conductive condition the fusewill burn out and render the circuit ineffective without disabling thespark plug to which it is connected.

Embodiments of the present invention will now be described, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic illustration of a conventional electrical ignitionsystem for a four cylinder combustion engine;

FIG. 2 illustrates a current versus time waveform for a spark generatedby a conventional circuit of the type illustrated in FIG. 1.

FIG. 3 is a general circuit diagram illustrating components which can becombined in a variety of configurations to form an embodiment of thepresent invention:

FIG. 4 illustrates an embodiment of the present invention incorporatingonly two of the components of FIG. 3, that is a capacitor and a parallelresistor;

FIG. 5 illustrates a current versus time waveform for a spark generatedby a spark plug connected in a conventional ignition system such as thatillustrated in FIG. 1 supplemented by a circuit as illustrated in FIG. 4connected between the coil and distributor;

FIG. 6 illustrates a second circuit in accordance with the presentinvention;

FIG. 7 illustrates a current versus time waveform for a spark generatedusing the circuit of FIG. 6;

FIG. 8 illustrates a further circuit in accordance with the presentinvention;

FIGS. 9 and 10 represent current versus time curves for sparks generatedusing the circuit of FIG. 8 but at different engine speeds;

FIG. 11 illustrates a further embodiment of the present inventionincorporated in a circuit of the type illustrated in FIG. 1;

FIG. 12 illustrates an embodiment of the present invention incorporatedin a conventional circuit but in a different configuration to that ofFIG. 11;

FIG. 13 illustrates an embodiment of the present invention used togenerate two substantially simultaneous sparks in a cylinder providedwith two spark splugs;

FIG. 14 illustrates the structure of a capacitor suitable for use inembodiments of the present invention;

FIGS. 15 to 19 illustrate the effect on output power of fitting acircuit in accordance with the present invention to a conventionalignition system;

FIG. 20 illustrates the effect on hydrocarbon output of fitting acircuit in accordance with the present invention to a conventionalignition system; and

FIG. 21 illustrates the effect on carbon monoxide output of fitting acircuit in accordance with the present invention to a conventionalignition system.

Referring to FIG. 1, this illustrates the basic components of aconventional coil-energised spark ignition system. Four spark plugs 1 to4 are connected between a distributor 5 and a source of fixed potentialindicated by the earth symbols. The distributor 5 houses a rotor arm 6driven in synchronism with the engine to which the ignition system isfitted. A high tension lead 7 is connected between the rotor arm and astandard ignition coil winding 8 which in turn is coupled to a source offixed potential indicated by the earth symbol. Thus when the rotor arm 6is adjacent a distributor terminal connected to one of the four hightension leads leading to the spark plugs, voltage induced in the coil 8is supplied to the respective spark plug and a spark is generated.

FIG. 2 illustrates the current versus time relationship for a sparkgenerated by a conventional system such as that illustrated in FIG. 1.There is an initial "brightline" capacitive discharge indicated by theline 9 but the spark terminates with a relatively ineffective inductiveflaring portion indicated by line 10.

Referring now to FIG. 3, this is a general circuit diagram of a range ofcomponents which can be incorporated in a circuit in accordance with thepresent invention. These components comprise a capacitor 11, a resistor12 in parallel with the capacitor 11, a voltage control discharge tube13, a series diode 14 and a parallel resistor 15. The reference numerals11 to 15 are used throughout the following description where appropriatebut it will be appreciated from the following description that the onlycomponents which must always be present in any embodiment of theinvention are a capacitor 11 and either a resistor 12 or a voltagecontrolled discharge device 13. Preferably the capacitor 11 isnon-linear, having a capacitance which reduces with applied voltageand/or a capacitance which reduces with operating temperature. Theresistor 12 may also be non-linear, preferably having a resistance whichfalls with applied voltage. The discharge tube 13 is provided simply toprevent unduly high voltages being applied to the capacitor 11 andtherefore does not normally affect the operation of the circuit. Thediode 14 is a normal diode capable of carrying for example 500 mA. Theresistor 15 is a simple linear resistor having a resistance of forexample 1 Mohm and a rating of 5 watts and 5 kV. The purpose of thecircuit illustrated in general form in FIG. 3 is to alter the currentversus time waveform from the conventional waveform as shown in FIG. 2so as to improve the performance of an internal combustion engine towhich the circuit is fitted.

Referring now to FIG. 4, this illustrates a first embodiment of thepresent invention. The capacitance of capacitor 11 decreases with theapplied voltage. Such characteristics are readily achieved with knownceramic disc capacitors, the relationship between capacitance andapplied voltage being represented by a smooth but non-linear curve. Inone practical implementation of the circuit of FIG. 4, the capacitanceof capacitor 11 was 1000 pF at 0 volts, 600 pF at 6 kV, and 300 pF at 12kV. The resistor 12 is also voltage dependent, having a resistance at 0volts effectively of infinity, a resistance at 6 kV at 12 Mohms, and aresistance at 12 kV or 1 Mohm.

Referring to FIG. 5, this illustrates the current versus time waveformfor a series of sparks generated as a result of introducing the circuitof FIG. 4 between the coil and distributor of a conventional ignitioncircuit such as that illustrated in FIG. 1. It will be seen that in theillustrated case three brightline sparks are generated each of which cancontribute to the efficiency of combustion. The less effective inductiveflaring part of the spark shown in FIG. 2 is substantially reduced. Thuswith the circuit of FIG. 4 the brightline spark is repeated andalternated. Tests have indicated that the circuit described withreference to FIGS. 4 and 5 aids combustion particularly in the case oflean fuel mixtures.

In greater detail, when the distributor connects the coil to one of thespark splugs through the circuit of FIG. 4, a primary winding (notshown) of the coil is broken by a conventional mechanism within thedistributor and a negative voltage spike of several thousand volts istransmitted through the capacitor 11 to the spark plug. When themagnitude of this voltage has risen sufficiently the gap defined by thespark plug is ionised sufficiently for a spark to be formed. Currentthen flows from the earth terminal of the spark plug through the circuitof FIG. 11, the distributor 5 and the coil 8 to earth. This current flowis indicated in FIG. 5 by the sharp negative current flow represented byline 16 and initiated shortly after the start of the current versus timeplot.

The current which passes through the capacitor 11 causes a voltage to bedeveloped across the capacitor. As this voltage rises, the currentdelivered to the spark plug falls and eventually the voltage developedacross the coil 8 is not sufficient to sustain a spark in the spark pluggap. Thus the current fails to zero. Once this has occurred, thecombined reverse bias voltage of the coil 8 and the capacitor 11 issufficient to re-ionise the gap defined by the spark plug but this timein the opposite direction. The capacitor then disharges through thespark gap and this is indicated in FIG. 5 by the line 17. Thus the sparkplug is alternatively ionised in one direction and then in the other andspark current flows in each of these directions.

Depending upon the engine configuration, the coil, the spark plug gaps,and the capacitance value of the capacitor 11, the current may ceaseafter one spark in each direction or more cycles of operation may besustained.

The resistor 12 has a resistance value sufficiently high as to havelittle impact on the united magnitude of the current flowing to thespark plug. The resistor 12 could have a stable rsistance, in which caseits purpose is simply to discharge the capacitor of any residual chargebetween successive energisations of the spark plugs. It is howeverpossible to simply dispense with the resistor 10. Results achieved witha simple capacitor circuit with no parallel resistor are describedbelow. It is however preferred to provide the resistor 12 such that itsresistance falls with applied voltage.

In the event of a malfunction with the circuit of FIG. 4 the voltageacross the capacitor 11 can build up to such a high level that thecapacitor can break down and fail to a low impedance condition. Thisdoes not prevent the circuit continuing to operate in a conventionalmanner, that is to say as if the capacitor 11 and resistor 12 wereabsent, but does make the circuit formed by capacitor 11 and resistor 12inoperative. To prevent such a high voltage malfunction occurring it ispossible to supplements the circuit as described below by connecting athreshold voltage discharge device 13 in parallel with the capacitor 11.The discharge device 13 will be rated to break down at a voltage abovethe normal operating voltage of the capacitor but below a voltage atwhich damage to the capacitor could occur. Thus generally the dischargedevice 13 will be inoperative but it is there to protect the capacitor11 if circumstances arise in which unduly high voltages are generated inthe coil. The discharge device 13 would be provided as an alternative toor in addition to the provision of a non-linear resistor such as avaristor in parallel with the capacitor. Varistors are available theresistance of which falls linearly with applied voltage for voltages ofa few thousand volts and the resistance of which falls rapidly at highervoltages, e.g. 12 Kv.

The capacitor 11 of FIG. 4 exhibits a large capacitance to thebrightline edges but its non-linearity with respect to voltage ensuresthat it shuts down the less effective inductive flaring components. Theresistor 12 protects the capacitor against overcharging. The effectivecapacitance exhibited by the circuit determines the frequency of thebrightline edges such that: ##EQU1## where L is the inductance of thehigh tension coil and C is the effective capacitance of the circuit.

The circuit of FIG. 4 can be used on all conventional vehicles subjectto its use not disrupting other control mechanisms. For example invehicles with engine speed counting mechanisms associated with theignition system, the multiple AC sparks per ignition cycle could disruptthe engine speed monitoring circuits.

Referring now to FIG. 6, this illustrates a further embodiment of thepresent invention in which the capacitor 11 is in parallel with adischarge tube 13 and in series with a diode 14. FIG. 7 illustrates thecurent versus time spark waveform assuming that the circuit of FIG. 6 isincorporated in a conventional ignition system either between the coiland the distributor or in each of the high tension leads leading fromthe distributor to the spark plugs.

The diodie 14 ensures that the circuit retains a DC spark. It producesan "echo" brightline discharge to improve the ignition properties. Theecho discharge is indicated by line 19 in FIG. 7. Again, the capacitor11 is voltage dependent to pass the brightline edge but also to reduceinductive flaring components. The circuit of FIG. 6 can be used with anyengine speed counting mechanism as the spark remains DC. The circuit ofFIG. 6 is suitable for use in lean burn engines.

Referring now to FIG. 8, this illustrates an embodiment of the presentinvention which is capable of reducing cyclical dispersions using staticcharge retention. The circuit of FIG. 8 comprises capacitor 11, resistor12, diode 14 and resistor 15.

Cyclic spark ignition dispersion is caused as a result of the spark notbeing of the correct intensity and duration given a particular enginespeed. Accordingly cyclic dispersion can be reduced by decreasing thespark intensity and duration at high engine speeds.

FIGS. 10 and 11 illustrate spark waveforms achieved by incorporating thecircuit of FIG. 8 in a conventional ignition system. FIG. 9 showing theresults at 2000 rpm and FIG. 10 showing the results at 5000 rpm. As canbe seen from FIGS. 9 and 10, the maximum spark current decreases withincreasing rpm as does the spark duration. The diode 14 does not affectthe first brightline edge indicated by line 20. The resistor 15decreases the rate of discharge of the capacitor 11 such that at highrpm the capacitor 11 cannot fully discharge. The small positive currentsindicated in FIGS. 9 and 10 result from current passing through theresistor. The capacitor 11 thus retains a static charge which isproportional to rpm. This acts as a barrier to the next spark andtherefore reduces its intensity. Thus the circuit matches the sparkshape, intensity and duration to engine speed.

Referring now to FIG. 11, this illustrates a further embodiment of thepresent invention incorporated in an otherwise conventional ignitionsystem of the type shown in FIG. 1. In the case of the circuit of FIG.11, however, the resistor 12 is mounted physically close to thecapacitor 11 so that the energy dissipated in the resistor 12 can beused to increase the temperature of the capacitor 11. The capacitor hasa capacitance which decreases with temperature. Again conventionalceramic disc capacitors are well known which exhibits suchcharacteristics.

With such an arrangement, the capacitance of the capacitor 11 reduces asthe power dissipation increases with engine speed, that powerdissipation increasing with engine speed. As the capacitance reduces,then so does the amplitude and duration of the spark. Thus as enginespeed increases the spark size reduces, and cyclic dispersion is reducedas a result of the temperature modulation of the non-linear capacitor.On the other hand, at lower temperatures the spark amplitude isincreased which is also beneficial.

With the arrangement of FIG. 11 it is necessary to mount the capacitor11 and resistor 12 on a heat sinking structure. Power dissipation in theresistor 12 is typically from 2 to 12 watts. The capacitor 11 can bearranged to change in capacitance from 1000 pF to 300 pF over atemperature range of the order of 100° C.

The circuit of FIG. 11 incorporates a series diode 14 which enables onlyDC spark generation. This can be advantageous under certaincircumstances, for example in the case of well maintained and well tunedengines. This approach reduces the temperature of the spark plug and therate of carbon disposition on the plug by reducing the length andmagnitude of the current waveform.

Thus the described circuits enable spark ionisation distribution in timeand space to be optimised. The circuits can be fitted as originalequipment or retrofitted to existing ignition systems. Cyclic dispersionin firing cycles at different engine speeds can be reduced. This canlead to improved power, reduced emissions and reduced predetonation,engine knocks and pinking.

The components can be fabricated from conventional material. For examplenon-linear resistors can be fabricated using silicon carbide (SiC).Capacitors can be fabricated using conventional ceramic material such asbarium titanate.

In the arrangements described with reference to FIGS. 1 to 11, thecircuit of the invention is connected in series with one or more of thespark plugs. The circuit is also applicable as an enhancer ofconventional DC sparks in a configuration such as that shown in FIG. 12,in which the same reference numerals are used where appropriate. In FIG.12, the high tension supply 8, lead 7 and the plugs 2, 3 and 4 areomitted to simplify the illustration. As shown, the capacitor 11,resistor 12 and discharge device 13 are connected in parallel between ahigh tension lead 21 connecting the plug 1 to the distributor 5 and afuse 22 which is connected to ground. With this arrangement, when thecoil primary is broken the current initially flows through the capacitor11 from ground until voltage builds up across the capacitor. The voltagebuilds up to a sufficient level to cause the plug 1 to spark andthereafter the charge on the capacitor 11 sustains the spark such thatthe magnitude and duration of the DC spark is enhanced.

In the arrangement of FIG. 12, if the resistor or capacitor 11 were tofail to a low impedance condition, the spark plug 1 would be in effectshort circuited and would be inoperative. If this was to happen howeversuch a high current would be drawn through the fuse as to exceed itsrating and as a result the fuse 22 would burn out. The circuit formed bycomponents 11, 12 and 13 would then be inoperative and the system wouldagain continue to operate in a conventional manner. Thus the systemfails safe in an operative condition.

Referring now to FIG. 13, this illustrates a further application of thecircuit shown in FIG. 12. In the arrangement of FIG. 13 two plugs 23 and24 are positioned in the same cylinder of an internal combustion engineand are intended to fire simultaneously. Such twin plug arrangements arewell known. Each of the plugs 23 and 24 is connected to a high tensionlead terminal 25 via a respective circuit, each of the two circuitscomprising a capacitor 11, a resistor 12 and a discharge device 13.Again when the coil primary is broken, current initially flows throughthe capacitors 11 to cause the plugs 23 and 24 to fire. This arrangementalso facilitates the possibility of out of phase sparks. A reverse sparkis then induced as a result of charge building up on the capacitors 11.This arrangement ensures that both plugs fire reliably and there is notendency for the firing of one plug to disable the firing of the other.Further charge storage could also be achieved by connecting a furthercircuit of the type illustrated in series with the high tension leadconnected to the terminal 25.

In the arrangement of FIG. 13, a diode could again be connected betweenthe terminal 15 and each of the circuits but this would produceuni-directional current through the plugs.

FIG. 14 illustrates the structure of one ceramic disc capacitor havingappropriate temperature and voltage characteristics. The capacitorcomprises a disc 26 of barium titanate secured between two terminals 27and 28 by a resin casing 29. Such a capacitor will typically have anouter diameter of 16.5 mm and an axial thickness of 10 mm. The capacitorhaving the dimensions illustrated in FIG. 14 has a capacitance at 12 kVor 380 picofarads.

Initial tests have conducted to assess the effect of connecting circuitsin accordance with the present invention in conventional ignitionsystems. The results of these tests are set out in FIGS. 15 to 21. Ineach of the test cases, the circuit was in the form of a simple ceramicdisc capacitor connected between a conventional ignition coil and aconventional distributor. The capacitor in each case was applied voltagedependent.

Referring to FIG. 15, this shows the relationship between engine speedand power output for a Ford Sierra car. The lower curve represents theresults with an unmodified ignition system and the upper curverepresents the results of fitting a capacitor in series with the coiloutput, the capacitor having a capacitance of 1000 picofarads at zeroapplied volts.

Referring to FIG. 16, this illustrates results obtained with a CitroenVisa vehicle running on a rolling road. Engine speed is represented byvehicle speed. The lower curve shows the results of an unmodifiedignition system and the upper curve shows the results with a modifiedignition system, a voltage dependent capacitor being connected in serieswith the output of the ignition coil and a resistor being connected inparallel with the capacitor. The capacitor had a capacitance of 500picofarads at zero applied voltage and the resistor has a resistance of5 Mohms at zero applied voltage.

Referring to FIG. 17, this illustrates the relationship between powerand engine speed in the case of a 1986 Renualt 11 GLS. Again the circuitused was a simple capacitor in series with the coil output. The lowercurve shows results before the circuit was modified and the upper curveshows results after modification.

FIG. 18 shows the results obtained with a Vauxhall Astra car. The lowercurve indicates performance with an unmodified ignition system and theupper curve shows the effect of connecting a capacitor in series withthe coil output. The capacitor used has a capacitance of 1000 Pf at zeroapplied volts.

FIG. 19 illustrates results obtained on a rolling road for a For Ganadacar. The lowr curve indicated power with an unmodified ignition systemand the upper curve indicates power after a voltage dependent capacitorwas connected in series with the coil output. The capacitor had acapacitance of 1000 pF at zero applied volts.

FIG. 20 illustrates the effects on hydrocarbon emissions. The uppercurve indicates emissions with an unmodified ignition system and thelower curve indicates emissions after a capacitor was connected inseries with the vehicle coiled output. The capacitor used had acapacitance of 1000 PF at zero applied volts.

FIG. 21 shows the effects on carbon monoxide emissions resulting formthe same vehicle and the same circuit modification as generated theresults of FIG. 20. The lower curve represents emissions with a modifiedcircuit and the upper curve emissions with the unmodified circuit. Theresults of FIGS. 20 and 21 were obtained from a conventional VauxhallAstra.

Thus, tests have shown that circuits as described with references to theaccompanying drawings operate in a particularly efficient manner toprovide improved combustion. These improved performance characteristicsarise from the circuit providing enhance brightline capacitive dischargecomponents of continuous rising and decaying edges and more advantageouscurrent waveforms. With circuits not incorporating a diode, ions impactboth plug electrodes thereby maintaining clean spark plugs. ACexcitation also produces better ionisation. This leads to better startup ignition. Where there are multiple capacitive rising surrent edgesthis will help ignite leaner fuel mixtures. Thus overall bettercombustion characteristics can be achieved giving improved enginecleanliness, reduced emissions and improved engine efficiency. Bysuitable pulse shaping, the circuit may also be used to produce currentwaveforms which lead to a substantial reduction in radio frequencyinterference emissions.

In summary, the invention can provide benefits including:

1. A high voltage circuit which produces a dual polarity spark from aconventional single polarity high tension coil source.

2. A high voltage circuit which enlarges brightline capacitivecomponents of a single polarity spark produced from a conventional hightension source.

3. A high voltage circuit which produces simultaneous twin, single ordual polarity sparks from a conventional single polarity high tensionsource to drive two spark plugs per cylinder arrangement without theneed for dual ignition systems.

I claim:
 1. An electrical circuit for connection to a high tension leadwhich is connected to a spark plug of a spark ignition internalcombustion engine, the circuit comprising a capacitor, the capacitanceof said capacitor when employed along being such that if a high voltagepulse is applied to the high tension lead, the voltage developed acrossthe capacitor and the charge stored by the capacitor are sufficient toinitiate and sustain an ignition spark, wherein there is a resistorconnected in parallel with said capacitor.
 2. An electrical circuit forconnection to a high tension lead which is connected to a spark plug ofa spark ignition internal combustion engine, the circuit comprising acapacitor, the capacitance of said capacitor when employed alone beingsuch that if a high voltage pulse is applied to the high tention lead,the voltage developed across the capacitor and the charge stored by thecapacitor are sufficient to initiate and sustain an ignition spark,wherein there is a voltage controlled discharge device connected inparallel with said capacitor.
 3. An electrical circuit according toclaim 1, wherein the capacitor is non-linear.
 4. An electrical circuitaccording to claim 1, wherein the capacitor is voltage dependent suchthat its capacitance reduces with increases in applied voltage.
 5. Anelectrical circuit according to claim 1, wherein the capacitor istemperature dependent such that its capacitance reduces with increasesin operating temperatures.
 6. An electrical circuit according to claim1, wherein the capacitor is fabricated from a ceramic material.
 7. Anelectrical circuit according to claim 6, wherein the ceramic material isbarium titanate.
 8. An electrical circuit according to claim 1, whereinthe resistor is non-linear.
 9. An electrical circuit according to claim8, wherein the resistor is voltage dependent such that its resistancedecreases with increases in applied voltaged.
 10. An electrical circuitaccording to claim 5, wherein the resistor is positioned such that heatgenerated in the resistor is transferred to the capacitor.
 11. Anelectrical circuit according to claim 1, wherein the resistor isfabricated from silicon carbide.
 12. An electrical circuit according toclaim 2, wherein the voltage controlled discharge device is a dischargetube.
 13. An electrical circuit according to claim 1, comprising a diodeconnected in series with the capacitor.
 14. An electrical circuitaccording to claim 1, wherein said circuit is for connection in serieswith a spark plug.
 15. An electrical circuit according to claim 14,wherein the circuit is connected between a high voltage pulse source anda distributor, a plurality of spark plugs being connected to thedistributor.
 16. An electrical circuit according to claim 1, whereinsaid circuit is for connection between a source of fixed potential and ahigh tension lead in series with a spark plug.
 17. An electrical circuitaccording to claim 16, wherein a fuse is connected in series with thecapacitor, the fuse being rated to burn out if the capacitor fails to alow impedance conductive condition.
 18. An electrical circuit accordingto claim 1, wherein a first said circuit is connected in series with afirst spark plug and a second said circuit is connected in series with asecond spark plug, the series connected first circuit and spark plug andthe series connected second circuit and spark plug being connected inparallel between a high tension lead and a source of fixed potential.19. An electrical circuit according to claim 18, wherein there is adiode connected in a high tension lead to which each of the said seriesof connected circuits is connected.